The invention relates to a method of heating glass, in which method glass is led through a tempering furnace, whereby the glass is heated from above and below, whereby air is blown at least onto the upper surface of the glass to heat the glass in such a way that air is sucked from the inside of the tempering furnace, the air being circulated in such a way that it is blown back onto the glass.
Further, the invention relates to an apparatus for heating glass, which apparatus comprises a tempering furnace comprising means arranged to support the glass and to form its conveyor and means for sucking air from the inside of the tempering furnace and for circulating the air to be blown back at least onto the upper surface of the glass.
When glass is heated by means of an oscillating roller furnace, the problem is that the edges of the glass curve upwards at the initial stage of the heating. This is due to the great heat flow onto the lower surface of the glass caused by the ceramic rollers used in the furnace at the initial stage of the heating cycle, compared with the heat flow onto the upper surface of the glass. Consequently, the edges of the glass curve upwards, the central area of the glass being easily affected by optical errors, and in addition, the glass becomes unevenly heated. When selective glasses are heated, the situation is particularly difficult, because selective glasses reflect heat radiation particularly intensively. Glasses with selective surfaces are usually heated in such a way that the selective surface faces upwards, whereby the heating of the upper surface of the glass is remarkably more difficult compared with the heating of the lower surface of the glass. Thus, the heating times of selective glasses are naturally longer than the heating times of ordinary clear glass, whereby the capacity of the furnace is typically rather low when selective glasses are heated.
FI patent 62043 discloses a method of preventing the curving of glass. In this method, the upper surface of the glass is subjected to a heat flow by means of forced convection, which heat flow compensates for the heat flow from the rollers below. The forced convection has been provided by blowing horizontal narrow air jets in the longitudinal direction of the furnace, which provide a turbulence effect of air with an injector effect onto the upper surface of the glass. The air jets have been achieved by taking pressurized air compressed by compressors from a compressed-air network outside the furnace. FI patent 83072 discloses a corresponding method, in which the air blown as air jets is also circulated through the lower section of the furnace, whereby the air is heated during this extra round. At the same time, the heat transferred to the air is taken from the lower part of the glass. In both of the methods the effect of the convection is rather low, whereby the method is rather ineffective. The air to be conveyed into the furnace is cold, so that it cools the furnace in its entirety, which increases the energy consumption of the furnace in total. Further, a problem is the uncontrolled discharge of the air to be blown from the furnace. Further still, the emphasis in the method is on intensifying the heating of the upper surface of the glass at the initial stage of the heating. Thus, the total heating time of selective glasses is long, because selective glasses are, in any case, primarily heated from the lower part of the glass applying the radiation principle.
EP publication 0897896 discloses a solution in which coated glass is heated by blowing air onto it from longitudinal blowpipes. The air to be blown is taken from a compressed-air network outside the furnace. The arrangement comprises a compressed-air tank which is provided with overpressure by means of a compressed-air compressor. Due to the compressed-air arrangement, the structure of the solution becomes complex and expensive. If cold air is blown into the furnace, it cools the furnace in its entirety, and thermal energy has to be directed into the furnace in some other way. Heating the air to be blown, in turn, requires a large amount of energy and capacity, so that what it comes to the energy economy, the solution according to EP publication 0897896 is, as a whole, poor. Further, the uncontrolled discharge of the air to be blown from the furnace is a great problem.
FI publication 962158 discloses a method in which the surfaces at the lower side of the glass are cooled at the initial stage of the heating cycle, and correspondingly, the heat transfer of the lower side is intensified at the final stage of the heating cycle by blowing hot air directly to the lower surface of the glass. FI publication 962162 discloses a solution, in which the heating resistors are dimensioned and their control implemented in such a way that the heating resistors are many times more efficient, whereby the heating of the glass at the initial stage of the heating cycle may be performed by utilizing the upper resistors only. The methods are very efficient and well-functioning, but it would be desirable, particularly when heating selective glasses, to make the heating time shorter.
A known solution is also what is known as a convection furnace, in which the intention is to heat glass by blowing hot air onto the upper and lower surfaces of the glass, as well as to the ceramic rollers. In such a solution, air is circulated in the furnace with blowers constructed inside the furnace, whereby the flow velocity of the air is increased, the aim being thus to increase the effect of the air on the surface of the glass. The air is blown at a pressure of approximately 0.005-0.01 bar. The air is heated in the solution either prior to the blower or after the blower. A problem of the solution is particularly the high manufacturing cost, and the slow speed of the heating due to the large mass of the air channels constructed inside the furnace, and uncontrollable heat expansions of the construction.
U.S. Pat. No. 4,505,671 discloses a solution in which glass is heated by blowing heated gas onto its upper and lower surfaces. The gas is taken from a separate gas source and heated with a separate heater. The solution consumes considerable amounts of gas from the gas source. Further, heating gas consumes energy. Increasing the amount of flowing gas and thus increasing the heat-transfer coefficient is in this solution rather difficult.
U.S. Pat. No. 4,059,426 describes a solution in which the glass is supported by gas jets and air is blown with a blower onto the surface of the glass sheet, the air being circulated back to the blower. However, this kind of solution does not allow the surface of the glasses to be subjected to a sufficient heat effect. Further, the publication discloses a solution providing air circulation inside a furnace by utilizing the Coanda phenomenon. The air circulation inside a furnace does not provide a sufficient heat effect on the surface of the glasses either.
Moreover, a solution is known in which the glass is heated in two steps. At the first stage, a lower temperature is used, whereby air having a temperature of about 300 to 400xc2x0 C. is circulated in the furnace by means of blowers. The air is blown directly onto the upper and lower surfaces of the glass and heated prior to the blowers. At the latter stage, the glass is heated using mainly radiation heating. In this solution, too, the problem has turned out to be the high cost of the air channel system constructed inside the furnace and of the blowers used in the solution. Further, the heating of the glass at the latter stage takes rather a long time, particularly when selective glasses are heated.
An object of this invention is to provide an improved method and apparatus for heating glass.
The method according to the invention is characterized by leading the glass through a tempering furnace by means of a conveyor consisting of rollers; pressurizing the air sucked from the tempering furnace by applying the compressor principle to an overpressure of over 0.1 bar relative to the pressure in the tempering furnace; leading the pressurized air by means of a pipe system into the vicinity of the surface of the glass located upon the rollers; and blowing the air substantially perpendicularly onto the upper surface of the glass.
Further, the apparatus according to the invention is characterized in that the apparatus comprises horizontal rollers, which are arranged to support the glass and to form its conveyor; a pressurization unit; a return pipe of the upper side; and blow pipes of the upper side, which blow pipes are arranged in the vicinity of the surface of the glass, whereby the return pipe is arranged to convey air from the tempering furnace to the pressurization unit, and the pressurization unit is arranged to pressurize the air conveyed from the tempering furnace to an overpressure of 0.1 bar relative to the pressure of the tempering furnace by applying the compressor principle, whereby the pressurized air is hot and arranged to be blown through the blow pipes of the upper side substantially perpendicularly onto the upper surface of the glass.
An essential idea of the invention is that glass is heated upon rollers in a tempering furnace from the upper and lower sides of the glass. At least the upper surface of the glass is heated with air jets directed substantially perpendicularly, i.e. at an angle of below 45xc2x0 relative to the perpendicular of the glass surface in such a way that the air has been directed by means of a pipe system into the vicinity of the glass, the air jets having been provided by sucking air mainly from the inside of the furnace and by pressurizing the air taken from the inside of the furnace to an overpressure of 0.1 bar relative to the pressure of the tempering furnace by applying the compressor principle. The idea of a preferred embodiment is that the lower surface of the glass is also heated in a corresponding way with hot air jets, which air jets have been provided by taking air mainly from the inside of the furnace and by pressurizing the air taken from the inside of the furnace to an overpressure of 0.1 bar relative to the pressure of the tempering furnace. The idea of a second preferred embodiment is that glass is also heated by means of electric resistors.
An advantage of the invention is that since the pressure level of the air is fairly high, a high discharge velocity is achieved for the air, and at the same time, a very high heat-transfer coefficient on the surface of the glass is achieved. Since the air blown is hot, air can be blown directly as far as onto the glass surface and air can also be blown until the end of the heating cycle of the glass. Further, owing to the high pressure level and the hot air, high heat-transfer coefficients can be achieved with a small amount of air, whereby the pipe system of the apparatus is small and simple and thus there are no risks what it comes to thermal movements. Since in the solution the air to be blown is taken from the inside of the furnace, the furnace has no problems that would be caused by the discharge of excessive air. Further, the amount of air and at the same time the heat-transfer coefficient can be increased basically without limits. Increasing the amount of air and the heat-transfer coefficient can be performed simply by increasing the size of the pressurization unit, whereby the heat losses of the furnace do not increase significantly. A significantly shorter heating time is achieved for the glass by means of the method according to the invention. Particularly when selective glasses are heated, the heating time can be made considerably shorter, because the solution according to the invention utilizes convection heating in a very efficient manner, and the radiation properties of the glass surface do not substantially weaken the effect of the convection heating. What is known as a heating profile can be created for the furnace by means of electric resistors, convection blowing having at the same time enabled the raising of the furnace capacity. Further, a furnace provided with a heating resistor is very easy to keep in balance compared with for example convection furnaces where the intention is to implement the heating with mere air jets. In such solutions, the channel system surfaces in the vicinity of the glass get cool compared with the rest of the area and may cause imbalance in the furnace. The solution is very easy to mount afterwards, because the apparatus and its pipe system are small in size and simple.